In optical transmission systems, cables, such as fiber-optic cables, are used to transmit information. In some systems, such as that shown in FIG. 1, cables 15 and 15a-d extend from an optical line terminal (“OLT”) 10 to one or more optical network units (“ONUs”) 20a-d. An optical signal(s) of a certain wavelength or group of wavelengths, indicated in the figure as λi, is transmitted downstream (downstream signal) from the OLT 10 to the ONUs 20a-d via the cables 15 and 15a-d. An optical signal of a certain wavelength or group of wavelengths, indicated in the figures as λj, is transmitted upstream (upstream signal) from the ONUs 20a-d to the OLT 10.
If damage occurs to one or more of the cables 15 and 15a-d, for example a break in a cable from inclement weather, rodents or otherwise, it may be desirable to determine the location of the fault (or break) along the cable, such that the cable may be repaired.
One way to determine the location of the break in the cable is shown in FIG. 2. An optical time-domain reflectometer (“OTDR”) 30 is installed coupled to the portion of the optical fiber cable 15 located near the OLT 10, and an OTDR signal reflector 40a-d is installed coupled to the portion of the optical fiber cable 15a-d located near each ONU 20a-d. An optical signal of a certain wavelength or group of wavelengths, indicated in the figure as λk, is sent downstream along the cables 15 and 15a-d to the ONUs 20a-d from the OTDR 30, and the OTDR signal reflectors 40a-d are configured to reflect the λk signal upstream toward the OLT 10, such that the reflected signal λk may be detected by the OTDR 30. If a break or fault occurs in the cable upstream of the OTDR reflector 40a-d, and downstream of the point at which the cable 15 extending from the OLT 10 is joined with individual optical cables 15a-d extending respectively to the ONUs 20a-d, the λk signal will not be reflected back to the OTDR 30 by the OTDR reflector 40a-d, which is downstream of the break. The distance between the OTDR 30 and each OTDR signal reflector 40a-d is very accurately known and stored in a database, such that based upon the information of the distance associated with each OTDR reflector 40a-d and the lack of a reflected λk signal from a particular OTDR signal reflector 40a-d associated with a corresponding ONU 20a-d, it may be possible to determine in which cable 15a-d between an ONU 20a-d and the point at which the multiple cables 15a-d extending to the respective ONUs 20a-d are joined to the single cable 15 extending from the OLT 10 the fault is located.
Examples of known OTDR signal reflectors 40a-d are shown in FIGS. 3 and 4. For example, in FIG. 3 an OTDR signal reflector 40a is shown as a Bragg grating 41a in an optical fiber cable 15a. In FIG. 4, an OTDR signal reflector 40a is shown as a thin film wavelength division multiplexer (“WDM”) filter 42a. Each OTDR signal reflector 40a is designed to allow λi and λj signals to pass therethrough with no or minimal attenuation, such that the λi and λj signals are transmitted between the OTL 10 and ONUs 20a-d with no or minimal degradation or power loss, while almost completely or completely reflecting the λk signals with minimal attenuation. However, in practice, some portion of the λi and λj signals incident on the OTDR signal reflector 40a-d is reflected. In particular, the reflection of the λi signals upstream toward the OTDR 30 is undesirable because such reflected λi signals may cause crosstalk or interference.
Therefore, it would be desirable, when performing OTDR in an optical network to detect cable faults or breaks, to provide a device that maximizes reflection of the λk signal while minimizing any reflection of other signals, such as the λi and/or λj signals, thereby increasing the accuracy with which an OTDR may help determine the particular location of a fault in an optical fiber cable of the optical network.